The pollution produced by the exhaust from internal combustion engines is increasingly of concern. These pollutants include hydrocarbon, carbon monoxide (CO), nitrogen oxide (NO.sub.x), and particulate emissions. The type and amount of emissions depend, among other things, on the type of engine and fuel system and on operating conditions. For example, diesel engines produce relatively low amounts of CO, but produce significant amounts of particulate matter in the form of soot, that is comprised of carbon, ash, that is comprised of inorganics, and polynuclear aromatic hydrocarbons (PAHs), that are condensed about the carbon nuclei of the soot. NO.sub.x emissions are also a significant problem for diesel engines.
NO.sub.x emissions arise from reactions occurring during the combustion process which involve nitrogen present in the combustion air (atmospheric nitrogen) or, to a lesser extent, bound in the fuel (fuel-bound nitrogen). NO.sub.x formation from atmospheric nitrogen is dependent on-the temperature at which combustion occurs. In general, the greater the temperature in the combustion chamber, the greater the resultant NO.sub.x emissions will be. Conversion of fuel-bound nitrogen to NO.sub.x depends on the amount and reactivity of the nitrogen compounds in the fuel and on the amount of oxygen present.
Among other known techniques, exhaust gas recirculation (EGR) has been successfully used to reduce NO.sub.x emissions in the exhaust stream from an engine. With EGR techniques, a portion of the exhaust is recirculated back into the engine. The exhaust gas replaces a portion of the combustion air in the engine, resulting in less oxygen available to enter into the reactions, and lowers the temperature at which combustion occurs. A lower concentration of NO.sub.x emissions in the exhaust gas stream results. Recirculation of ten to fifteen percent of the exhaust gas can lead to a fifty to sixty percent reduction in NO.sub.x emissions.
However, there is a tradeoff between NO.sub.x reduction and particulate emissions. The more air that is displaced by the recirculated exhaust gas, the less O.sub.2 is introduced into the engine, which leaves more fuel unburned. Unburned or partially burned fuel increases the concentration of CO and particulate emissions in the exhaust stream. Although CO emissions from diesel engines are relatively low and a small increase caused by EGR can be tolerated, the same is not true of particulate emissions; no increase in particulate emissions of diesel engines is environmentally desirable. In addition, particulates returned to the engine in the recirculated exhaust gas can cause extensive damage. The particulates are abrasive, and get into the engine oil, causing wear on the piston rings, valves, and other parts of the engine. For example, Bender et al., U.S. Pat. No. 5,138,835, discloses a diesel engine exhaust gas recirculation system, but damaging particulates are returned to the engine with the recirculated exhaust gas. Particulates are especially damaging to turbochargers or superchargers. If a diesel engine were equipped with such devices, at least ninety-nine percent of the particulates must be removed if the gas is to be recirculated.
Attempts have been made to filter recirculated diesel exhaust gas. However, these attempts have not fully obviated the problems of preventing engine damage from particulates in the recirculated gas, especially if equipped with a turbocharger or a supercharger, while also reducing NO.sub.x and particulate emissions from the exhaust gas stream. For example, U.S. Pat. No. 4,924,668, to Panten et al., discloses an exhaust gas recirculation system in which a filter for the recirculated gas is placed parallel to the exhaust flow, rather than directly in the recirculation line, to prevent the filter from clogging. However, the exhaust flow out of the vehicle is not filtered at all.
Porous ceramic and other filters have been used to capture unwanted particulate matter in the form of soot, ash, and PAHs condensed about the carbon nuclei of the soot, which are entrained in the emission stream of diesel engines. The soot is "sticky" and adheres quite readily to the walls defining the pores of the ceramic and other filters. However, prior art filters typically have particulate filtration efficiencies of significantly less than 95 percent and typically in the range of 50 to 80 percent; the passage of more than five percent of the particulates through the filter is enough to damage the engine. Also, with the prolongation of filtration, the soot so accumulates in the filters as to obstruct the pores. An obstructed filter induces a back pressure in the exhaust line which can affect engine operation and reduce in the effective throughput of the filters, necessitating the cleaning or replacement of the filters.
Thermal regeneration to remove the accumulated soot from the filters is known, such as by embedding resistive filaments in the ceramic matrix that oxidize the accumulated soot when energized. However, because hot spots tend to be formed thereby that cause thermal failures in the ceramic, not only is care required to prevent degradation of the filter matrix in the locale of the hot spots, but also degraded filters must be periodically monitored to ensure that they comply with the clean air emission standards. Fine ceramic particles can also be eroded and travel downstream, where they can cause damage to the exhaust system piping or to the engine. Further, the PAHs entrained in the diesel exhaust condense at and around 200.degree. to 400.degree. C. Filters which employ thermal regeneration techniques are generally located at the diesel exhaust manifold close to the engine and typically operate at temperatures well above the boiling point of the PAHs, which makes them generally unsuited to unburned PAH emission control or use in a recirculation line. For example, U.S. Pat. No. 4,462,379, to Tsuge et al., discloses a filtered exhaust gas recirculation system using a thermally regenerable filter located close to the engine. See also U.S. Pat. No. 4,356,806, to Freesh, which is directed primarily toward exhaust gas recirculation systems for use with gasoline engines, in which filters are located in the recirculation line close to the engine, where it is possible that filters suitable for filtering diesel exhaust could be damaged by engine heat or thermal regeneration or that PAHs could revolatilize. Moreover, thermally regenerated filters are prone to failure by melting and cracking of the ceramic matrix during the high-temperature regeneration periods.
Another alternative to thermal regeneration of the soot filters is aerodynamic regeneration using pulses of compressed air flowing through the trap in a direction opposite to the exhaust. In commonly assigned U.S. Pat. No. 5,013,340, entitled "Rotating Diesel Particulate Trap", incorporated herein by reference, soot is continuously removed by so rotating a particulate trap that, while one sector thereof is exposed to diesel exhaust flowing in one direction, another sector thereof is exposed to a counter flowing stream of high-velocity (high-mass) air provided either by a fan or a compressed air tank. The filter is rotatably mounted by a bearing assembly and is driven by an electric motor or belt connecting an axle to the drive shaft of the diesel engine whose exhaust is to be filtered, and rotary seals are provided to prevent cross-contamination of the exhaust and cleaning air streams.